The evidence that schizophrenia may involve infection by a virus (or viruses) has been indirect. This evidence includes the phenomenology of schizophrenia (insofar as it may be mimicked by some viral encephalitides), epidemiological factors (including a predominance among patients with late winter/early spring births, a north-south gradient, and occasional clustering of cases), and indirect laboratory evidence (gliosis in some neuropathological studies, spinal fluid protein abnormalities, and abnormalities in cell-mediated immunity). The discovery of the human retroviruses, HTLVI, HTLVII and HIV, now also known to affect the CNS, together with the development of new techniques in human retrovirology, made it possible to investigate the role of this class of viruses in the etiology of schizophrenia. Cultures of peripheral lymphocytes of patients with chronic schizophrenia were established and tested for the retrovirus- specific enzyme reverse transcriptase.